U.S. Pat. Nos. 2,921,632, 3,036,639 and 3,182,614 as well as a copending patent application Ser. No. 204,750, filed Nov. 7, 1980, disclose various forms of packers such as may be used in an oil or gas well in association with other well tools for servicing or preparing a well for production purposes. In such a well, a packer may be used to create a seal against the flow of pressure fluid in the annular space between tubing in the well and the well casing. Various mechanical arrangements are employed in such packers for anchoring the packer at a preselected position vertically in the well and to effect the desired sealing. For example, in the packer disclosed in the aforementioned U.S. Pat. No. 3,036,639, vertically spaced sets of slips mounted on upper and lower expander heads are wedged radially outward to anchor the packer in the well casing by vertical manipulation of a tubular mandrel carrying both the expander heads and the slips. As the expander heads are moved toward each other, a sealing assembly which includes an elastomeric sealing element is squeezed between the expander heads so as to expand radially and seal against the inside wall of the casing. At opposite ends of the sealing element, expandable metallic sealing rings engage the inside wall of the casing so as to keep the elastomeric material in the element from extruding between the expander heads and the casing and thereby reducing the effectiveness of the seal. Between the expander heads and the packer mandrel, close clearance tolerances may be maintained to avoid the loss of elastomeric material.